Radiant burners or surface burners of the type according to the preamble have a mixing chamber in which a gas mixture of fuel gas and an oxygen carrier gas is produced. Connected to the mixing chamber is a burner plate with passage channels through which the gas mixture flows out of the mixing chamber and is burned.
The passage channels in the burner plate for the throughflow of the gas mixture from the mixing chamber side to a combustion side are so narrow that the individual flames forming on the exit side cannot flash back into the mixing chamber. A flash-back of the flames through the passage channels into the mixing chamber is prevented if the diameter of at least sections of the passage channels is smaller than the so-called extinction distance (or also quenching distance) of the combustion. The quenching distance is the distance from the fuel gas outlet within which no reactions take place and a flame cannot spread as the released combustion enthalpy is absorbed by the surrounding burner material and conducted away and the reaction chains are terminated. However, the quenching distance is not an absolute value but depends i.a. on the composition of the fuel gas, the fuel-gas temperature and the wall temperature.
With a radiant burner, the thermal output produced by the combustion is to be distributed evenly over a large area. For this, the material of the burner or burner plate is heated by the flames of the gas combustion until it glows and delivers an effective heat radiation to the material to be heated. If the flames burn as individual flames over the burner plate, the material is heated only weakly and inefficiently. To achieve an effective heating of the burner material, the flame is to burn as close as possible to and in close contact with the material. For this, the flame is preferably moved into the burner plate either by designing the latter porous and producing a blanket of flames in the porous material or by allowing the combustion to proceed in channels (combustion channels) inside the burner plate.
For example, a burner plate for a surface burner is known from DE 100 28 670 in which, on the exit side on the combustion side, channels with an enlarged cross-section compared with the passage channels, in which the combustion takes place, connect to passage channels for the fuel gas the diameter of which is smaller than the quenching distance of the combustion. For this design, the object of the invention in DE 100 28 670 was to produce a burner plate which makes possible a dramatic reduction in specific thermal output in order to use the burner to heat e.g. plastic material indirectly over a large surface area to low temperatures of only 100 to 300° C. For this it is necessary for the average surface temperature of the burner plate to be reduced to well below 900° C. without incomplete combustion resulting or the flame going out.
In the case of a high fuel gas throughflow to produce a high heat flux density, the individual flames burn on the exit side surface of the burner plate. When the heat flux density is reduced, they retreat progressively and pass into the combustion channels as their diameter is larger than the quenching distance of the combustion. In the case of a very low heat flux density, the flames remain at the transition zone between the passage channels and the enlarged cross-sections as the diameter of the passage channels is smaller than the quenching distance of the combustion. The specific thermal output of the burner according to DE 100 28 670 can thereby be very greatly reduced.
The described design has the disadvantage that, if a high fuel gas throughflow for a high radiant power and burner temperature is desired, the flames emerge from the combustion channels at the surface of the burner plate, whereby the radiant power falls and the flame is unprotected against flows and turbulences, which can result in the flame going out.